1. Field of the Invention
This invention relates to a means for detecting the relative positioning of a member, such as a wafer, on a support base, and is especially adaptable to ion implantation equipment.
2. Description of the Prior Art
State of the art ion implantation equipment has automatic wafer pick-up-and-place capability using robotics. State of the art ion implantation equipment includes the NV Series Ion Implanters, manufactured by Eaton Corporation, for example. Typically, one robotic arm will pick up wafers from a wafer cassette and position the wafers on a rotating table or disk before the implantation process. Another robotic arm is typically provided to remove the wafers from the disk and place the wafers onto another wafer cassette after the ion implantation process is completed. The wafer is held in place on a base within a set of retaining pins by a clamping arm which is pivotally connected to and movable toward and away from the base. A clamping arm is necessary to secure the wafers because the disk rotates during the ion implantation process at least 1,000 rpms. In order for wafers to be positioned on the disk and within the retaining pins on the base, the clamping arm must be moved away from the base. Movement of the clamping arm is typically actuated by a pneumatically-controlled clamp lifter. The clamp lifter comprises an air cylinder and a vertically movable elongated plunger. Air causes the plunger to move out of the air cylinder and through a slot located through the disk at a position which is aligned with the clamping arm. The plunger contacts the bottom side of the clamping arm, thereby causing the clamping arm to move away from the base as the plunger continues its vertical movement out of the air cylinder.
A great degree of accuracy is necessary to position the wafers within the set of retaining pins because the retaining pins are so located relative to each other such that a wafer only has minimum freedom of movement within the pins after the wafer has been placed. To that end, robotic arms fail to consistently achieve the degree of accuracy necessary. Therefore, a means for detecting when a robot arm fails to properly position a wafer within the set of retaining pins is necessary. The most common conventional means for detecting the relative positioning of a wafer on ion implantation equipment, such as the NV Series Implanters, uses a fixed photosensor to detect light when a wafer is mispositioned by the robotic arm. During proper operation, a robot arm picks up and places a wafer 20 into position on a base 21 within a set of retaining pins 22, as illustrated in FIGS. 1a, b. The retaining pins 22 protrude up vertically from base 21. A clamping arm 23 is pivotally connected to base 21 for movement toward and away from base 21. After the wafer 20 is placed by the robot arm, clamping arm 23 is moved toward base 21 to make contact with wafer 20 at points 26, 27 thereby clamping wafer 20 in place. A fixed photosensor 24 is provided which is mounted on a different base than wafer mounting base 21 and is shown in FIG. 1b. Photosensor 24 emits light in a continuous path. When wafer 20 is properly placed and clamping arm 23 clamps the wafer in place, end face 25 of clamping arm 23 is lower than the light path of photosensor 24. As long as the light path is undisturbed, the wafer pick-up and placement operation will continue.
Frequently, the equipment fails to operate properly because the robot arm is prone to drift out of adjustment and as a consequence, the robot arm improperly places wafer 20 onto the retaining pins 22 to some extent, as illustrated in FIG. 2a. Clamping arm 23 will sit higher on wafer 20 at one of the clamping points 26 when wafer 20 is improperly placed than when wafer 20 is properly placed within the retaining pins 22. Photosensor 24 should detect the improper placement because clamping arm 23 should be high enough to disturb the fixed light path, thereby reflecting some light back into photosensor 24. If clamping arm 23 disturbs the light path and photosensor 24 detects some light reflected off of end face 25 of clamping arm 23, it signals the robot arm to halt the wafer placement operation, thereby signifying a wafer has been improperly placed onto the retaining pins 22. In practice, clamping arm 23 may not disturb the continuous light path when wafer 20 is improperly placed onto retaining pins 22, because clamping arm 23 is not raised high enough to ensure that the light path is disturbed and some light is reflected into photosensor 24, as illustrated in FIG. 2b. Therefore, photosensor 24 will not signal the robot arm to halt its operation. Incidences such as this are attributable to machining runout and the tolerance levels achieved during machining component parts of the equipment, such as base 21, retaining pins 22, and clamping arm 23 and includes the eccentricity in the base 21 rotating mechanisms. These tolerance levels can be much greater than the displacement distance of clamping arm 23 when wafer 20 is improperly placed. Therefore, a displaced clamping arm 23 under these circumstances is frequently undetected by photosensor 24. As a result, an improperly placed wafer 20 left undetected causes destruction and disintegration of the wafers and spreads wafer particles throughout the process chamber during subsequent wafer processing. Downtime due to undetected mispositioned wafers could be up to 30 hours in a manufacturing/processing facility and could incur tremendous costs.